1. Field of the Invention
The invention concerns a casting compound suitable for casting an electronic module, in particular a large-volume coil such as a gradient coil.
2. Description of the Prior Art
Gradient coils of a magnetic resonance apparatus are formed of three sub-coils with which magnetic field gradients are generated in the three spatial directions. The X-coils and Y-coils are typically fashioned as saddle coils; the Z-coil is realized by means of a peripheral winding. The individual coils can be constructed both from bundled individual conductors and by separating structures in an electrically conductive plate (advantageously made of copper or aluminum) generated with suitable processing methods with, the remaining material representing the coil winding. The coil windings produced according to the different methods are then glued to an electrically insulating support plate and formed into the shape of a half-cylindrical shell, for example. The individual coil layers are mounted successively on a cylindrical mandrel. Additional components of the coil structure are insulation and reinforcement layers; one or more cooling layers composed of plastic tubes to conduct a cooling fluid (typically water); and possibly items known as shim coils. Additional layers in the coil structure are, for example, secondary windings that serve to externally shield the magnetic field generated by the primary coils.
The complete coil structure is cast with a casting compound (a casting resin based on epoxy resin), with which it is intended that all conductor interstices are impregnated without cavities and bubbles. A multitude of requirements are imposed on the casting compound (thus the casting resin). It must have a sufficiently low processing viscosity in order to be able to fill or penetrate all conductor interstices so as to be free of voids and bubbles. It should exhibit a high modulus of elasticity in order to be able to ensure a high overall rigidity and a positionally accurate fixing of the individual windings of the coil. It should also have good thermal conductivity in order to enable effective heat transport from the conductor structures (which generate heat during operation) to the cooling layer. The glass transition temperature (and therefore also the heat forming resistance) should be as high as possible in order to have an optimally constant property profile in the usage temperature range. The coefficient of thermal expansion should be as low as possible, preferably close to that of the other materials (copper conductors, insulation layers, etc.) that are used in order to minimize mechanical stresses (and thus the formation of cracks and delaminations in the coil composite that are due to these stresses) given heating due to operation or cooling from the curing temperature at which the casting resin is cured. In particular, the cracking resistance should be very high. The cracking resistance is characterized by a high critical stress intensity factor K1C associated with a high critical fracture energy G1C. The partial discharge resistance should also be high in order to avoid a damage to the mold material during the service life. The dielectric loss factor should be low.
Heat-curing casting resins based on epoxy resin are typically used as casting compounds, specifically for large-volume coils. Such a casting compound typically contains approximately 65% filler, for example quartz powder, aluminum oxide or wollastonite with particle sizes in the micrometer range, meaning that the filler consists of microparticles.
A casting compound and a magnetic coil cast with such a casting compound are known from EP 1 850 145 A2. This casting compound is mixed (compounded) with microparticle fillers and inorganic nanoparticles. A good crack resistance based on a combination of a high critical stress intensity factor K1C with a high critical fracture energy G1C can be achieved with these inorganic fillers or filler combinations. The filler addition produces positive alterations of the aforementioned properties of the casting compound mold to be cured and the heat curing resistance, the fracture toughness, etc. An optimally high volume fraction of filler would be appropriate for these reasons. Particularly the flow behavior of the prepared casting compound, however, is drastically impaired by the filler content. The higher the filler content, the poorer the flow behavior, and therefore the property of being able to impregnate the given conductor interstices, etc. without voids and bubbles. A compromise with regard to the casting resin composition is thus always sought. Overall there is a need for a casting resin that exhibits an optimally high crack resistance—in particular at a high glass transition temperature—with very good flow properties.